As shown in FIG. 1, a common approach to humidity control in homes and buildings in many parts of the country, such as in the Midwestern United States, is to mount a humidistat 10 on a return air duct 12 at the furnace 14. This is done, in many instances, in order to simplify the installation of the humidity control equipment. A humidistat is a conventional device that measures humidity and which controls humidification equipment associated with the home or building.
It has been previously thought that a duct mounted sensor provides a good average measure of the relative humidity level in the home or building. However, in many instances the temperature and the relative humidity percentage (%RH) in the duct is different from the temperature and %RH in the living/working space(s) of the home/building. The humidity sensor, which may be a nylon element or electronic sensor, associated with the humidistat, senses the %RH of the air in the duct at the temperature in the duct. This will cause an error in the control point if the temperature and humidity in the living/working space is different from that in the duct.
For example, if the temperature in the living/working space is 72.degree. F. and the furnace is located in an unfinished basement, which is generally cooler that the rest of the building, it would not be uncommon for the temperature in the duct to be substantially lower than the temperature in the living/working space, such as 65.degree. F. or lower. Thus, if the control is set to 40% RH, the actual humidity level in the living/working space would only be 31%.
Further, the periodic cycling of the circulation fan associated with the furnace causes temperature and humidity fluctuations within the duct. During periods when the fan is not running and the air is not moving through the return air duct, it would not be uncommon for the temperature in the duct to be five to ten degrees higher or lower that the temperature in the living space. This temperature difference would change the relative humidity by as much as 16%RH. Thus, a humidity control relying upon the sensing of temperature or humidity within the duct would be inaccurate due to the variation of temperature and humidity within the duct due to fan cycling.
In addition, during periods when the thermostat of the space is set back a large amount, for instance from about 72.degree. F. to about 60.degree. F., the humidity level within the space will not be adequately controlled to achieve a comfortable level for the set temperature.
Although the %RH and temperature may temporarily fluctuate, the dew-point temperature, which is a function of %RH and temperature, remains relatively constant. Thus, the dew-point temperature within the living/working space would be generally equal to the dew-point temperature within the return air duct, regardless of the temperature and humidity differences between the two locations. Therefore, it would be desirable to be able to control humidification equipment based upon dew-point temperature, thereby providing more accurate control of the humidification equipment and reduce unnecessary operation thereof.